Electronic transformer/inductor devices and methods for making same

ABSTRACT

The present invention relates to the methods of construction for inductive components of, preferably, ferromagnetic materials such as inductors, chokes, and transformers when used as an integral part of the fabrication of PCB&#39;s or FLEX&#39;s. In one preferred embodiment, holes are formed through a ferromagnetic substrate and plated with conductive material. The arrangement of these holes, and the subsequent design that ensues, will form the inductive components within the plane of the media in which the device is formed; using the substrate for a magnetic core. By using this approach, the inductive components can be miniaturized to physical sizes compatible with the requirements of modem surface mount technology (SMT) for integrated circuitry (IC). This process also allows these components to be fabricated using mass production techniques, thereby avoiding the need to handle discrete devices during the manufacturing process. In another preferred embodiment, a series of thin, concentric high permeability rings are etched on a substrate to provide high permeability transformers and inductors having minimal eddy current effects.

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional Application No.60/234,636 filed Sept. 22, 2000 entitled “ElectronicTransformer/Inductor Devices And Methods For Making Same” and U.S.Provisional Application No. 60/237,356 filed Sept. 27, 2000 entitled“Electronic Transformer/Inductor Devices And Methods For Making Same.”

FIELD OF THE INVENTION

The present invention relates to inductive components and methods formanufacturing these components.

BACKGROUND OF THE INVENTION

Inductive components are commonly fabricated using ferromagnetic coresand windings of insulated electrical wire. The ferromagnetic cores aretypically toroidal cores, rod cores, or assemblies made of a lower Eshaped ferromagnetic part and a ferromagnetic cap connecting the threelegs of the E such as shown in FIG. 1.

The toroid and rod cores are manually or automatically wound with theinsulated copper wire to form a number of multiple turn windings for atransformer or a single winding for an inductor. The assembly is thentypically encapsulated to protect the wires. The circuit connection ismade by the solder termination of the wires as required by theapplication. This approach has high labor costs because of individualpart handling. It has large variability in electronic parameters such asleakage inductance, distributed and inter-winding capacitance, andcommon mode imbalance between windings because of the difficulty inexact placement of the copper wires.

The E shaped and encompassing cap assembly of FIG. 1 is made into aninductive component by manually or automatically winding copperinsulated wires around the legs of the E as required. Either gluing orclamping the cap in place and final encapsulation completes thissubassembly. Similarly, the circuit connection is made by means ofsolder termination of the wires as required by the application. Not onlydoes this device have the limitations of the toroid and rod core, asmentioned above, but also it generally is a much larger device. Becausethe cap is a separate device the magnetic paths have a resistance ofnon-ferromagnetic gaps between the E and the cap reducing the efficiencyof the transformer.

Power transformers constructed as shown in FIG. 1 have the furtherdisadvantage that the heat resulting from the resistance losses in thewindings is not easily dissipated because the E core and cap isolatethese windings from a heat sink.

SUMMARY OF THE INVENTION

The preferred embodiments of the present invention provide inductors andtransformers and methods of manufacturing these devices which offer verysignificant advantages over the state-of-the-art. These inductors andtransformers connected in accordance with this invention have a numberof applications in the electronics, telecommunication and computerfields. In one preferred embodiment described below, a rectangular slabof ferromagnetic material is encapsulated between printed circuitry. Aplurality of through holes (vias) are drilled through or formed duringmanufacture of the slab from the top face of the slab to the bottom faceof the slab, the number of holes corresponding to the number of desiredturns of the windings. This embodiment utilizes Ampere's Law in a verynovel manner to form a transformer, inductor, or the like within thecircuit board rather than the use or assembly of discrete inductivedevices to the circuit board. Thus, the windings are not insulatedelectric wires. Rather, the holes through the slab are made electricallyconductive by through hole plating or the like and electrically connectwith the printed circuits encapsulating the slab. This pattern of platedthrough holes and the printed circuitry form the inductor andtransformer windings with the core of the inductors and transformersbeing the drilled or formed slab of ferromagnetic material. Thisembodiment provides substantial improvements, particularly infabricating high frequency inductors and transformers.

In another preferred embodiment described below, the core of theinductors or transformers comprises cores formed by a multi-layer seriesof thin concentric ferromagnetic metal rings supported on a suitablesubstrate such as a flex circuit (FLEX) or printed circuit board (PCB).Through holes proximate these concentric ring cores provide electricalconnection with printed circuitry to provide the inductor andtransformer windings. This embodiment enables construction of highpermeability inductors and transformers having minimal eddy currenteffects. Inductors and transformers so constructed have particularapplication for miniature low frequency power supplies.

In addition to the advantages described above, the preferred embodimentshave a number of additional significant advantages. These include:superior heat removal, outside connections that are more accessible tosimplify electrical connection, shorter flux paths to increase magneticperformance, simpler fabrication, interconnections that are moreintegrated, smaller inductive devices, superior performance, andexcellent manufacturing repeatability.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus summarized the general nature of the invention and itsessential features and advantages, certain preferred embodiments andmodifications thereof will become apparent to those skilled in the artfrom the detailed description herein having reference to the figuresthat follow, of which:

FIG. 1 is a conceptual illustration of a prior art ferromagnetic E core,with a matching ferromagnetic cap;

FIG. 2A is a top view of a conventional toroidal transformer;

FIG. 2B is a side view of a conventional transformer;

FIG. 3A is a top view of a representation of a “virtual” toroidaltransformer;

FIG. 3B is a side view of the virtual transformer of FIG. 3;

FIG. 4 shows a top view of other preferred embodiments of a virtualtransformer;

FIG. 5 shows an array of 70 cores laminated onto a large panel of FLEXwith the top FLEX layer removed to show the individual cores;

FIG. 6 is an enlarged side view showing top and bottom FLEX laminated toan individual core slab;

FIG. 7 shows a cross-section of a via hole in an individual slab;

FIG. 8 shows an example of a PCB prepreg with an array of 25 holes tohouse 25 cores;

FIG. 9 is an enlarged side view of an individual core showing top andbottom PCB laminated to the core;

FIG. 10 an enlarged cross-section of a via hole in an individualferromagnetic slab;

FIG. 11 is an enlarged cross-section of a via hole filled with screenedconductive paste;

FIG. 12 illustrates the invention's heat dissipation characteristic byimproved surface area to volume ratio;

FIG. 13 shows a metal toroidal core illustrating the manner in whicheddy currents are generated;

FIG. 14 illustrates a plurality of core laminations formed by etchingconcentric rings of ferromagnetic metal;

FIG. 15 is a is a enlarged view of one of the core laminations of FIG.14;

FIG. 16A is a is a cross-sectional view showing a plurality of stackedcore laminations;

FIG. 16B is an enlarged view of one of the core stacks of FIG. 16A;

FIG. 17A is a cross-sectional view showing the stack of FIG. 16A afterthe top and bottom printed circuits have been added;

FIG. 17B is an enlarged view of one of the core stacks of FIG. 17A;

FIG. 18A is a cross-sectional view after plated through via holes havebeen drilled through the laminated structure of FIG. 17A; and

FIG. 18B is an enlarged view of one of the core stacks of FIG. 18A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates a typical prior art transformer with a toroidal core30. For simplicity this transformer has two windings of insulated wire:a two-turn winding 32 and a four-turn 34 winding. Each turn 36 encirclesthe material of the core 30 such that when electrical current is passedthrough one winding an encircling magnetic flux path 38 flows within thecore 30. FIG. 2A illustrates the windings 32, 34 passing through thecenter of the core 30, and around the outside of the core 30.

Preferred embodiments of the present invention have a very differentcore and winding arrangement. In one of these preferred embodimentsshown generally in FIG. 3, slab 50 of ferromagnetic material has a topsurface 52 and bottom surface 54, and, shown in cross-sections, twoouter holes (vias) 56 and one inner hole (via) 64 within the slab 50. Asdescribed below, for miniature inductors and transformers, the slab 50is advantageously a thin layer of ferrite having a relatively highresistivity.

FIGS. 3A and 3B show a “virtual” toroid transformer constructed inaccordance with one of the preferred embodiments of the invention usingthe slab 50 with eight outer via holes 56 and six inner via holes 68(not all of which are shown in FIG. 3B). Conductors 58 are formed on thetop 52 and bottom 54 surfaces of the slab. The conductors have pads 60for connection to other devices, or circuitry. As described below, theseouter and inner vias 56, 68 are plated through from top to bottom and inelectrical contact with the conductor 58 so that a complete electricalcircuit extends between, for example, pad 60 and pad 60′. As describedbelow, a virtual toroidal transformer 62 is thus formed having acircular path 62.

FIG. 4 illustrates shows a ferromagnetic slab 70 with two independentvirtual cores 72, 74 residing on the same slab 70. The secondtransformer 74 illustrates another embodiment of the invention in whichthe vias are rearranged in specific patterns. This rearrangement forms arectangular virtual toroid 75 with an inner rectangle 76 and an outerrectangle 77.

The windings 32, 34 as shown in FIG. 2 are inductively coupled togetherby way of the core 30. The core 30 provides a magnetic flux path whichcouples the first winding 32 to the second winding 34 thereby generatingan electrical voltage at the second winding when there is a voltagepresent at the first. This is the basic theory of a transformer, whichtransforms energy from one winding to another. The major portion of themagnetic flux is constrained between the inner 40 and outer 42 radius ofthe core.

Ampere's law constrains the flux path independent of the shape of thecore; therefore, as discussed in detail below, it is not necessary toactually fabricate the donut shape of the toroid to create devices thatbehave similarly.

Fabrication as Part of a FLEX

One method of fabrication is to embed a multiplicity of ferromagneticslabs (cores 90) within the top and bottom layers of FLEX 92 circuitssuch as shown in FIGS. 5, 6 and 7. Copper circuit patterns 92corresponding to the desired windings are formed on an epoxy sheets 1,10which are glued to the top and bottom surfaces 112, 114 of the slab byadhesive 115. The cores 90 are thus contained in the circuits 92 by alamination process. Via holes are formed through the composite layers ofFLEX 92, and the cores, to form a connection between the top FLEX 116circuitry to the bottom FLEX 117 circuitry, as illustrated in FIGS. 6and 7. Filling vias with conductive inks and standard industry platingprocesses are preferred methods used to make the connections for largenumbers of virtual cores simultaneously. The cross section of thisconstruction is shown in FIGS. 6 and 7.

Fabrication as Part of a PCB

Another method of fabrication shown in FIGS. 8, 9, and 10 is to embed amultiplicity of ferromagnetic pieces 150, between the top 170 and bottom172 layers of PCB circuits. FIG. 8 shows an array of holes 154 in a PCBadhesive, or prepreg array 156. This prepreg 156 panel is formed toaccommodate each core piece 150. As shown in FIG. 9, after the pieces150 are inserted into the holes 154 a top section 170 and a bottomsection 172 of PCB are laminated to the array 156. The pieces 150 arethus contained by the lamination process which sandwiches the pieces 150between two sheets of epoxy. Via holes 190 are formed through thecomposite layers of PCB 192, and pieces 150 , to form a connectionbetween the top PCB 194 circuitry and the bottom PCB 196 circuitry. Vias190 filled with conductive inks 198 or standard industry platingprocesses are advantageously used to make the connection for a largenumber of cores simultaneously. The cross section of this constructionis similar to the FLEX 117 construction shown in FIGS. 6 and 7. Themajor difference is due to the inflexibility of the PCB material, whichdoes not conform to the individual pieces 150.

Fabrication without a FLEX or PCB

Another method of fabrication is shown in Figure 11 in which amultiplicity of cores 210 are retained on a carrier 212. Each core 210is molded with appropriate holes 214. Standard industry conductive inkscreening processes are then used to form the circuits on the top 215and bottom 216 of the cores 210 while simultaneously filling the holes214 to make the required connection between the top 215 and bottom 216sides.

Novel Employment of Ampere's Law

The embodiments of the invention described above, with conductive viasthrough the magnetic slab, employ Ampere's Law in a very novel manner.The vias are formed in such fashion as to allow a flux path to existbetween two windings formed on the substrate. Thus, as shown in FIG. 3A,any enclosed path that falls within the inner vias 68 will encompasszero net current, therefore such paths will have no magnetic flux. Anypath that encompasses the outer vias 56 will also encompass zero netcurrent because the inner holes 68 have an equal but opposite currentflow to the current flowing in the outer holes 56 creating zero magneticflux in the region encompassing the outer vias 56. However, the enclosedpaths between the inner and outer vias 68, 56 will have a net magneticflux due to the enclosure of the inner vias 68. Other paths thatpartially enclose inner or outer vias 68, 56 will not have significantmagnetic flux because the flux will select the shortest physical path,similar to electrical current. Thus, this configuration will behave verysimilar to the toroid of FIG. 2, and is shown as a virtual toroid 62.

It will be apparent that the proper selection of via holes enables manydifferent shapes of virtual cores and arrangements of cores onsubstrates. Thus, many independent magnetic circuits can be constructedon the same substrate. Because of this, it is possible to construct morecomplex circuits than simple inductors and transformers by theappropriate placement of vias and circuit conductors on the top andbottom surfaces 52, 54 of the ferromagnetic slab 50 shown in FIG. 3.Using, for example, processes employed in conventional PCB and FLEX(flexible circuit boards) industries (photo-deposition, etching, andplating) multiple components such as resistors, capacitors andintegrated circuits can be placed on the same substrate to formmicro-miniature circuit assemblies.

Inductors and transformers useful for high frequency circuits such asare used for radio frequencies, typical ranges being 100 KHz to 100 MHz,can be constructed in accordance with the foregoing embodiments. Theferromagnetic slab 50 is advantageously formed of a thin layer offerrite material having typical permeabilities in the range of 100 to10,000 and resistivity in the range of 1,000 ohm/cm to 109 ohm/cm.Typical ferrite compositions include ferric oxide and alnico. Suchferrite materials have a sufficiently high resistivity such that theplated vias through the slab are insulated one of the other. Thetransformers and inductors so constructed are adapted forminiaturization. They eliminate the need for complicated pins orlead-frames. Thus, a slab 1.5 inches long, 1 inch wide and 0.05 inchesthick with 0.03 inch diameter vias can provide the core for two or moretransformers. The ferromagnetic slabs may be very small. Surface pads onthe top and bottom surfaces form the connections, and can be surfacemounted directly to PCB's, thus reducing the footprint of the device andmaking more room for other components. The plotted windings aresubstantially in two parallel planes. Therefore the windings of a ten(10) layer planar transformer device, a typical application, can bereduced in overall height by a factor of five (5). The ferromagneticslab may be very thin, e.g., 0.05 inches, so that the inductors andtransformers of the invention may be constructed substantially in onevery thin plane instead of a three-dimensional E core constructionfurther reducing the overall height by a large factor.

Further Preferred Embodiments of Transformer/Inductor Devices HavingHigh Flex Densities and Minimal Eddy Current

Many inductive devices such as low frequency power transformers requirecores having relatively high relative permeabilities typically in therange of 10,000 to 100,000. However, the improvements afforded by thepreferred embodiments are applicable to lower and higher values, e.g., arange of 1,000 to 1,000,000. Certain metals and metal alloys providethese high flex densities including steel, iron, silica iron, 78permalloy, Mumetal, purified iron, and supermalloy. Although these highflex densities can offer distinct advantages in constructingtransformers and inductors, the low resistivity of the metals allowinduced eddy currents to flow which counteract the benefits of thehigher flux densities. The induced eddy currents 300 caused by themagnetic flux flowing in a metal core are illustrated in FIG. 13.Present day transformers/inductors that use metal as a core normallyreduce these eddy currents by constructing the toroid or E core out oflaminated metal E strips, with each strip separated by some type ofdielectric bonding material. The entire E core contains many such stripsto form the full core. By means of this configuration the eddy currentis limited to the cross section area of each strip. As described below,a significant feature of this invention is to further reduce the coresection areas.

The fabrication of one embodiment of this invention enabling use offerromagnetic metal for the core material is illustrated in FIGS. 14-18wherein a flex circuit or printed circuit board 290 supports a series oflaminated thin metal annular rings formed on FLEX or PCB and separatedby dielectric sheets. Plated via holes within the center and outside ofthe annular rings and plated complete the electrical turns around thecore. As described below, this embodiment substantially minimizes eddycurrent by substantially reducing the cross sectional area of eachlaminated core section.

A plurality of core laminations are formed by first laminating thesheets of ferromagnetic metal to a PCB or FLEX 290 and then etching awayportions of the ferromagnetic sheet to form a pattern of a plurality ofclosely spaced, narrow continuous core segments. Thus, FIG. 14 shows anindividual layer of PCB or FLEX 310 with 16 etched core arrays. It willbe understood that the lamination and etching processes known in the artwill generally permit manufacture of more than 16 such arrays dependingupon the size of the array and pattern. Advantageously, the core arrays315 are etched using well known double-sided processes so that identicalarrays are formed on both the top and bottom of sheet 310.

An enlarged view of a single core array 315 is illustrated in FIG. 15which shows an array having 16 concentric ferromagnetic electricallyconductive metal rings 320 a-320 p insulated from each other by therespective etched out spaces or voids 325 a-325 o. Likewise, the area330 outside the array 315 and the area 335 within the innermost ring 320are void of magnetic material. This invention, however, is not limitedto a concentric ring array and it will be apparent to those skilled inthe art that other core arrays may be constructed such as a series ofsuccessively larger squares or rectangles insulated one from the other.

The next fabrication step is to stack a plurality of the PCB and FLEXlayers 310 with the arrays 315 substantially in alignment. As shown inFIG. 16A, core arrays of concentric rings 315 a-315 h are stacked one ontop of each other with the core patterns on each layer in alignment. Theresult is the fabrication of a plurality of high flux carrying metalcores having very small eddy current areas. Thus, the thickness of theoriginal sheet used to etch the arrays 315 can be very thin, typicallyin the range of 0.0005″ to 0.010″ inches. The concentric rings can beetched using conventional PCB or FLEX (FPC) etching techniques to verynarrow widths on the order of 0.002″ to 0.003″ . As a result, referringto the cross-sectional eddy current producing areas of the core are verydrastically reduced in size.

As part of the stacking process, a thin layer of dielectric material 340is placed adjacent to the top surface of each etched concentric ringarray 315. Typically, an epoxy material is used. This dielectric sheetand the dielectric sheet supporting the etched ferromagnetic rings maybe of different materials. Representative materials include epoxies andacrylics manufactured by Dupont and Rogers Corp. for manufacturing ofPCB boards and FLEX. Epoxies and prepregs (and epoxies with glass) aregenerally used to construct PCB boards and acrylics are generally usedto manufacture FLEX. During the laminating process, the voids 325, voids330 and voids 335 shown in FIG. 15 are filled with dielectric material340 shown in FIGS. 18A and 18B.

As described above, the electrical windings of the preferred embodimentsof this invention are advantageously provided by conductive through holevias in contact with printed circuitry on both sides of the corestructures. The fabrication steps for windings of the embodiments ofFIGS. 14-18 is shown in FIGS. 17A, 17B, 18A and 18B.

Referring to FIGS. 17A and 17B, additional layers of copper 350, 355 arerespectively laminated on the top and bottom surfaces along with twoadditional layers of dielectrics 360, 365 separating the copper surfacesfrom the etched metal surfaces.

The completed structure is illustrated in FIG. 18A and 18B with viaholes 370 drilled through the entire laminated array. These vias arelocated proximate to, but typically not in contact with the lowresistivity ferromagnetic rings so as to electrically insulate thewindings turns provided by the plated vias. These holes 370 are thenplated with a electrically conductive material, typically copper.Conductive inks and conductive pastes within the via holes may also beemployed. The copper layers 350, 355 are then etched to form circuitpatterns in electrical contact with the plated through holes 370 forforming windings around the concentric ring core arrays 315.

The embodiment shown in FIGS. 18A and 18B illustrates, for simplicity ofillustration, a small number of via holes 370 a, 370 b, 370 c and 370 dfor each transformer. It will be apparent to those skilled in this artthat the embodiments of FIGS. 14-18 can have multiple windings by addingadditional through holes. If necessary, additional copper layers may bestacked on layers 350, 355 to provide the requisite connections toadditional through holes.

Individual transformers and inductor devices are extracted from thelaminated array of FIGS. 18A, 18B by the usual methods of “die” cuttingor routing the parts from the array. Each such device can be used as areplacement for the traditional inductive devices shown in FIGS. 1 and13. Also, because the etched metal core is part of an array such asshown in FIG. 16, it can be interconnected to other components.

The Advantages of the Preferred Embodiments

One Piece Core:

In E core construction, as shown in FIG. 1, a gap forms between the Ecore and the cap which can not be avoided. Most transformers use an Etype core that requires one half of the core be joined to the otherusing, for example, epoxies and clamps. These processes aretime-consuming, introduce losses, and cause variances in the parametersof the devices due to the resulting gap between the E core and the cap.In contrast, the cores of the preferred embodiments of this inventionare a continuous piece, thereby providing improving transformerefficiency. The one-piece design also eliminates the need to join twoseparate pieces together in a separate processing step.

If an intentional gap is desired in the embodiments of FIGS. 14-18 toavoid magnetic saturation, a separation can be etched in each of theconcentric rings shown in FIG. 15. Such etched gap rings eliminate thelarge variations of the traditional mechanical separation of the Ecores.

Reduction of Eddy Currents:

Inductors and transformers constructed in the manner of FIGS. 14-18offer superior performance with much less eddy current by segregatingthe metal lamination in two directions. This results because thepreferred embodiment shown has a core which is both thinner thanconventional laminated cores by virtue of the fact (a) that the sheetsof metal, from which the rings 320 are etched, can be much thinner usingPCB or FLEX fabrication materials and (b) the individual insulated rings320 may be made very narrow. Since eddy currents are proportional to thesquare of the segment cross section area, the preferred embodimentsdramatically reduce eddy currents compared to traditional methods ofmaking transformers or inductors. For example, referring to traditionalE core shown in FIG. 1, the metal laminates of this core cannot beseparated in two directions because the strips would fall apart orsimply not have mechanical integrity.

Surface Mount:

Windings formed in accordance with the preferred embodiments can beformed into surface mount leads without the need for separate lead-frameconstructions, complicated pinning or end plating.

Interconnection:

Because the etched transformers/inductors are manufactured employingidentical processes used to manufacture PCB's or FPC's, the transformerscan advantageously be an integral part of the power supply or circuitassembly thereby reducing the physical size, reducing the connections,and, in general, making the assembly more compact and smaller. Circuitcomponents can be placed directly above or below the etched transformer,using the transformer area as the carrier for the balance of thecircuitry so that the area of the entire circuit would be as small asthe area of the transformer.

Magnetically Sound:

Cores constructed in accordance with the preferred embodiments offer amore efficient flux path with fewer losses than traditionaltransformers. These characteristics more closely resemble a toroid indesign and function. The magnetic flux path is shorter than comparabletransformers using traditional cores such as E-Cores and PQ Cores.

Size:

The preferred embodiments can be made smaller because they do notrequire complicated pins or lead-frames. Surface pads on the top andbottom surfaces form the connection themselves and they can be surfacemounted directly to PCB's thus reducing the footprint of the device andmaking more room for other components. Windings are in 2 planestherefore the windings of a ten- (10) layer planar transformer device, atypical application, can be reduced in overall height by a factor offive (5). The “core” is in one plane instead of a three-dimensional Ecore construction further reducing the overall height by a large factor.

Cost:

The preferred embodiments can be made from flex circuits and much lessexpensive to manufacture than multi-layer planar windings. Alsoeliminating and the need for lead-frame's, potting, and cap gluing thusmaking the device easier to manufacture.

Heat Removal:

A significant feature of inductors and transformers constructed inaccordance with the preferred embodiments of the invention is that theheat generating windings of are not buried within an assembly or woundon top of each other as in traditional transformers nor are they stackedtogether as in planar transformers. Instead, the plated windingssubstantially reside on the top and bottom planes of the transformer orinductor device. This layout offers superior heat dissipation with notrapped heat buried within windings. The PCB can be advantageouslyattached to a heat sink, separated only by a thin solder mask typicallyonly 0.005 inches thick, placing half of the windings in thermal contactwith the heat sink, thereby offering a superior surface area to heatratio. FIG. 12 shows one example of a large surface area 230 forexcellent heat removal directly mounted to a heat sink 232 such ascopper and aluminum.

While the invention has been described herein with reference to certainpreferred embodiments, these embodiments have been presented by way ofexample only, and not to limit the scope of the invention. Accordingly,the scope of the invention should be defined only in accordance with theclaims that follow.

1. (canceled)
 2. A miniature inductor/transformer having minimal eddycurrent effects comprising: a stack of a plurality of substantiallyidentical thin concentric ferromagnetic rings respectively separated bytheir dielectric layers; a first printed circuit and a second printedcircuit on opposite sides of said stack of the concentric ferromagneticrings; and conductive via holes through said stack in electrical contactwith said printed circuits, the axis of said via holes beingsubstantially parallel to the center axis of said concentric rings.
 3. Aminiature inductor/transformer comprising: a thin layer of ferromagneticferrite material on a substrate; and a plurality of plated vias throughsaid ferrite material, said vias providing electrical windings of saidinductor/transformer.
 4. A method for making a miniatureinductor/transformer comprising: forming a thin layer of ferromagneticon a thin sheet of insulating material; laminating said sheet havingsaid layer of ferromagnetic material between first and second thinsheets; forming through holes through said laminated sheets; platingelectrically conductive material within said through holes, forming aprinted electrical circuit on said first and second thin sheets inelectrical contact with said conductive through holes, said printedcircuitry providing a portion of an electrical winding of theinductor/transformer.
 5. The method of claim 4, wherein said sheets areprinted circuit boards.
 6. The method of claim 4, wherein said sheetsare flex circuits.
 7. The method of making a plurality of miniatureinductor/transformers comprising: laminating a plurality of spacedferromagnetic cores with top and bottom layers of circuitry.
 8. Themethod of claim 7, wherein said top and bottom layers are sheets of flexcircuits.
 9. The method of claim 7, wherein said top and bottom layersare printed circuit boards.
 10. The method of claim 7, wherein vias areformed in said ferromagnetic cores during fabrication of said cores fromferrite material.
 11. The method of claim 7, wherein vias are formed bydrilling through said ferromagnetic cores after fabrication of saidcores.
 12. The method of claim 7, wherein said cores are formed withinthrough holes, and using a conducive ink process to form, circuitry onthe top and bottom of said cures and through said holes.
 13. A methodfor making a miniature inductor/transformer comprising: forming a firstthin sheet having a printed electrical circuit thereon,; forming asecond thin sheet having a printed circuit thereon; laminating saidsheet having said layer of ferromagnetic material between said first andsecond thin sheets; forming through holes through said laminated sheets;and plating electrically conductive material within said through holesin electrical contact with said printed circuitry on said first andsecond thin sheets, said conductive through holes providing a portion ofan electrical winding of the inductor/transformer.
 14. The method ofclaim 13, wherein said sheets are printed circuit boards.
 15. The methodof claim 13, wherein said sheets are flex circuits.
 16. (canceled) 17.(canceled)
 18. (canceled)
 19. (canceled)
 20. (canceled)
 21. (canceled)22. (canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. A methodfor making devices for use in printed circuit board design comprising;printing a circuit pattern onto said circuit board; forming holes whichgo through said circuit board; filling said holes with a conductivematerial; and using the connections and said pattern such that a deviceis created which uses said circuit board as a magnetic core.
 27. Themethod of claim 26 further comprising the introduction of aferromagnetic substrate into said printed circuit board, and theformation of said holes through said substrate.
 28. An inductor ortransformer comprising: a slab of magnetic material having a series ofspaced holes therethrough; an electrically conductor material withinsaid holes, and electrical printed circuits located on the top andbottom surfaces of said slab respectively in electrical contact withsaid conductive material.
 29. A method for making a miniatureinductor/transformer having minimal eddy current effects comprising:etching a thin sheet of ferromagnetic metal to form a plural array often or more concentric narrow continuous rings of said ferromagneticmetal; stacking four or more of said arrays separated by a dielectricmaterial to form a plurality of cores, said cores so constructed havinga very small cross-sectional area defined by the thickness of the sheetof ferromagnetic material and the width of said concentric narrow ringsso as to minimize the eddy current effect; laminating said stack ofarrays between copper sheets; forming said copper sheets into printedcircuits; forming vias through said printed circuits proximate saidlaminated ferromagnetic arrays; and plating said vias in electricalcontact with said printed circuitry to form electrical windings.
 30. Amethod for making a miniature inductor/transformer comprising: forming athin layer of a ferromagnetic on a thin sheet of insulating material;etching said thin layer to form ten or more thin discrete continuousferromagnetic members; forming a first thin sheet having a printedelectrical circuit thereon forming a second thin sheet having a printedcircuit thereon; stacking four or more of said sheets having said etchedferromagnetic material members between said first and second thinsheets; forming through holes through said laminated sheets; and platingelectrically conductive material within said through holes in electricalcontact with said printed circuitry on said first and second thinsheets, said conductive through holes providing a portion of anelectrical winding of the inductor/transformer.